1. Field of the Invention
The present invention relates to a method of forming SiO.sub.2 thin films of high purity and, more specifically, to a method of forming a high purity SiO.sub.2 thin film which may be used, for instance, as an interlayer insulator between, e.g., aluminum wiring, a passivating film, a step coverage material, or a buffer material for a semiconductor device, and as an insulating layer for a magnetic head.
2. Description of the Related Art
Hitherto, SiO.sub.2 thin films have often been used as dielectric layers and passivating layers of solid-state devices in the preparation of semiconductor devices. Among the various methods of forming SiO.sub.2 thin films, one method has been examined from the viewpoint of planarizing the surfaces of the relevant devices. In this method, a solution of a silicone compound in an organic solvent is applied to the surface of the device while the device is being rotated. The resultant coating is dried and then subjected to heat treatment whereby the decomposition and the evaporation of organic substances proceed together with a polymerization reaction to form an SiO.sub.2 thin film.
In a method in which an organic solvent solution of a silicone compound is applied to the device being rotated, the decomposition and the evaporation of organic substances proceed together with the polymerization reaction upon the drying of the device after the coating process and subjecting the device to heat treatment. In order to achieve a thin film with increased density, the heat treatment is effected at 1100.degree. C. Thus, heat treatment at a high temperature of 1100.degree. C. is necessary with this method, in order to obtain a SiO.sub.2 thin film with good quality.
Another method is adopted from the view point of employing a low temperature as the film forming temperature. In this method, an SiO.sub.2 thin film is formed in a plasma CVD (chemical vapor deposition) process using, as raw materials, a nitrogen oxide gas and a silane gas diluted with a hydrogen or argon gas.
More specifically, in this method utilizing plasma CVD, a substrate heated to a temperature of 300.degree. to 400.degree. C. is placed in a vacuum chamber, and silane and nitrogen oxide which serve as raw materials are introduced into the chamber along with gases used for dilution, such as hydrogen, helium, and argon. While the internal pressure is maintained at a pressure on the order of 1 Torr, the gases within the chamber are subjected to a glow discharge in which plasma energy causes the generation of silicon ions, oxygen ions, neutral radicals and the like, which are then deposited on the substrate to form an SiO.sub.2 thin film.
With a method utilizing plasma CVD, the composition ratios of the silicon and oxygen have to be adjusted, and, in addition, optimal gas pressures, partial pressures, gas flow rates, and discharge power must be attained in order to achieve a further improvement in the quality of the film. Thus, this method requires a series of complicated operations for controlling the quality of the film (see "Hakumaku Handbook (Thin Film Handbook)", page 235, published by K. K. Ohm, 1983).
The above-described method also encounters the following problems. Namely, it is impossible to prevent the SiO.sub.2 thin film being formed from becoming contaminated by impurities such as part of the diluent gases or materials forming the chamber wall and so forth. Further, since the deposition rate of a film is usually 2 to 300 .ANG. per minute, the formation of a thin film takes a relatively long time. Still further, since residual stress of the film will be generated to the extent of about 5.times.10.sup.9 dyne/cm.sup.2, cracks may be formed. Finally, step coverage is inadequate.